1. Field of the Invention
The present invention relates to polyurethane adhesives and/or sealants which are used as filleting compositions, and, more particularly, to polyurethane adhesives and sealants, based on urethane reactants, which can be stored as one-component premix materials.
2. Description of Related Art
Many different adhesives and/or sealants have been developed for use in connection with electronic assemblies. In one particular assembly step, the adhesive is injected by syringe or other means along the edge of a component and sculptured into a fillet, such as with a spatula, to secure it in place and to aid in the heat-sinking of the component This operation is referred to as "filleting" and the adhesives so used are referred to as "filleting compositions". The latter must have good usable work life, good thermal characteristics, high thixotropy, and reasonable extrudability. Filleting adhesives are used to enhance thermal transfer for cooling of electronic components.
One-component materials are preferred over two-component materials in order to avoid the quality problems inherent with weighing, mixing, and degassing.
Frozen premix rigid epoxies and frozen premix polysulfides are known. However, the epoxies are too rigid and brittle for many applications. For example, breakage and warpage problems are common when dissimilar materials are bonded with epoxies. Thermal expansion mismatches cause stress due to the rigid joining of the adherends. Rigid epoxies are too strong to allow disassembly of the electronic components for rework or alteration.
Polysulfides have a "rotten egg" odor, are extremely weak, chemically react with silver particulate fillers (excluding electrically conductive adhesives), and are bad outgassers and source of contaminants. Moreover, heat stability is poor.
Polyurethane elastomeric adhesives and sealants which are based upon polybutadiene polyols have advantages over other polymeric types due to superior adhesive, mechanical, chemical, and electrical properties. This family of urethanes has a thermal range consistent with most terrestrial applications. They remain flexible to low temperatures down to -94.degree. F. (-70.degree. C.). Being elastomeric, they are stress-dissipating adhesives for dissimilar adherends. They are adjustable in mechanical strength from weak to very strong (depending on formulation), can be electrically conductive, and can be non-outgassing.
However, polyurethanes cannot presently be stored for practical periods of time in one-component frozen form. Many such polyurethanes are stable for only one month, and even the best formulations are stable for only three months. Practical periods of storage are deemed to be a minimum of six months storage at -40.degree. F. (-4.degree. C.).
Another drawback of urethanes is that non-sag rheological properties have not been possible while retaining a void-free consistency. Polyurethanes are sensitive to moisture-containing materials in that a chemical reaction with isocyanate liberates CO.sub.2 gas. Common thixotropic agents, such as conventional fumed silica, bring moisture into this system and exacerbate this problem.
Thus, there remains a need for a one-component urethane formulation that can be stored for long periods of time at low temperatures and, when thawed, can be applied to form non-sag fillets, which, upon curing, form elastomers that are free of voids, tears, or bubbles.